The invention pertains to AGC amplifier circuits and more particularly to multiple stage AGC amplifier circuits having a differentially connected transistor pair in each stage and possessing dc feedback for providing optimum amplifier gain and AGC characteristics.
These circuits commonly employ a relatively simple resistive feedback connection for applying the requisite balancing dc bias voltage to the transistors of each transistor pair. An amplifier circuit of this form is disclosed in U.S. Pat. No. 3,396,750, entitled, "AM-FM Receiver Having Improved Bias Supply Circuit", William Peil and Robert J. McFadyen, issued Feb. 3, 1976. The circuit with a resistive dc feedback connection will maintain the proper bias and perform admirably for a continuously operated amplifier. However, there are numerous applications in which a gated amplifier operation is desirable. One such application is in an intrusion alarm system disclosed in a copending application for U.S. Letters Patent entitled "Ultrasonic Intrusion Alarm System", the invention of Robert J. McFadyen and John Zeigler, filed concurrently with the present patent application and assigned to the same assignee. Neither the referred to resistive dc feedback circuit nor any other known dc feedback connection is suitable for a gated amplifier operation because the requisite stable dc bias voltage cannot be continuously maintained from one gated "on" period to the next. The present invention overcomes this limitation of the prior art circuits.